In subsea oil exploration, a so-called “Christmas tree” is sometimes placed on the wellhead. The wellhead, itself, may be located many thousands of feet below the sea surface. Such a “Christmas tree” commonly has various valves, including a blow-out preventer (“BOP”) to prevent the unintended discharge of hydrocarbons into the sea.
With existing applications, however, such valves are often operated hydraulically by providing pressurized hydraulic fluid from a surface ship down to the wellhead. (See, e.g., U.S. Pat. Nos. 4,864,914 and 7,424,917 B2.) In some cases, the wellhead may be as much as ten-thousand feet below the sea surface. The pressure drop experienced in transmitting pressurized fluid through a pipe for some ten-thousand feet can be very large, and can reduce the usable pressure available at the sub-surface wellhead. Other devices rely on surface-powered power sources. (See, e.g., U.S. Pat. Nos. 7,159,662 B2, 4,095,421 and 3,677,001.)
In many cases, it is desired to provide such a blow-out preventer with a fail-safe feature. Should there be a failure, for whatever reason, an actuator will close a valve to prevent hydrocarbons from being released from the wellhead into the sea. With a tethered system, a failure of the surface-to-wellhead umbilical, may itself result in the loss of pressure sufficient to operate the actuator.
Some subsea devices have been developed, but these often are actuated by a compressed spring. (See, e.g., U.S. Pat. Nos. 7,108,006 B2, 6,125,874 and U.S. Re. 30,114.)
Accordingly, it would be generally desirable to provide a submersible electrohydraulic actuator that would be not require such an umbilical connection to a source of power (i.e., hydraulic or electrical) on a surface ship, and which would provide a source of fluid pressure that would be available to operate the valve in the event of a sensed failure or on command.